1. Field of the Invention
The present invention relates to optical-frequency-stabilized light sources with high accuracy and stability of the optical frequency, used for the reference optical frequencies of optical-frequency-multiplexed transmission systems, local light sources for transmitters or receivers, and light sources for high-resolution frequency analysis.
The present application is based on Patent Application No. Hei 9-39525 filed in Japan, the content of which is incorporated herein by reference.
2. Conventional Art
In the present specification, pulsed light having an optical frequency which changes by a standard frequency interval each standard period of time shall be referred to as "reference pulsed light", and a pulsed sequence of the reference pulsed light shall be referred to as a "reference pulsed light sequence".
Japanese Patent Application No. Hei 4-32274 (Japanese Patent No. 2567776) describes an optical-frequency-stabilized light source which receives an optical signal in which such a reference pulsed light sequence is repeated at a predetermined period, capable of generating a continuous beam of an arbitrary optical frequency based on the optical signal, wherein the accuracy and stability of the optical frequency can be controlled to 10 MHz or less.
FIG. 7 is a block diagram showing an example of the structure of such a conventional optical-frequency-stabilized light source, and FIG. 8 is a time chart showing an example of the timing of various control signals in this optical-frequency-stabilized light source. In FIG. 8, signals (1)-(3) correspond to parts (1)-(3) in FIG. 7.
The optical-frequency-stabilized light source 700 shown in FIG. 7 stabilizes the optical frequency by using the output beam from the reference frequency generating device 500 shown in the same drawing.
In FIG. 7, the reference frequency generating device 500 comprises a pulsed light source 600 composed of a single-wavelength light source 16 capable of generating light of optical frequency .function. and an optical switch 17, and an optical loop circuit connecting an 10 optical multiplexer 18, an optical switch 19, an optical delay line 20, a wavelength-converting element 21, an optical amplifier 22 and an optical splitter 23.
The pulsed light source 600 introduces pulsed light having a pulse width of T.sub.w and a period of T.function. into the optical loop circuit via the optical multiplexer 18. This pulsed light circulates through the optical loop, as a result of which the above-described reference pulsed light sequence is output from the optical splitter 23. At this time, the optical switch 19 is opened before introducing the next pulsed light into the optical loop, so as to reset the pulsed light which is circulating. Then, the next pulsed light is introduced into the optical loop. As a result, a reference pulsed light sequence is repeatedly output at a predetermined period.
Additionally, the optical-frequency-stabilizing light source 700 comprises an optical multiplexer 1, an optical detector 2, a rectifier 3, a time measuring circuit 4, a frequency measuring circuit 5, a CPU 706, a local light source 707 and an optical splitter 8.
At the optical multiplexer 1, the reference pulsed light output from the reference frequency generating device 500 is combined with a portion of the output beam of the local light source 707 split by the optical splitter 8. The optical detector 2 detects pulsed beat signals between the reference pulsed light and the local light. The time measuring circuit 4 synchronizes with the standard period of the reference pulsed light sequence and measures the time of detection of the beat signals. Furthermore, the frequency measuring circuit 5 measures the frequency of the beat signal. At this time, the optical frequency of the local light source 707 can be calculated from the set of measured data (time, frequency). By feeding back these calculated values to the local light source 707 via the CPU 706, it is possible to output CW light of an arbitrary optical frequency.
When using an optical-frequency-stabilized light source as described above as an optical frequency standard device or a light source for high-density wavelength-multiplexed communications, the stability and accuracy of the optical frequency should preferably be high. In conventional optical-frequency-stabilized light sources, when the reference pulsed light to the optical-frequency-stabilized light source is cut off due to a fault or the like on the transmission line to the reference frequency generating device or the optical-frequency-stabilized light source, it becomes impossible to control the frequency of the optical-frequency-stabilized light source, thereby degrading the stability and accuracy of the optical frequency.
Additionally, since the beat signal frequency measuring time is short in conventional optical-frequency-stabilized light sources, there is a large dispersion in the detected frequencies. This can result in divergence of control if direct feedback is applied to the local light source, thus worsening the degree of stability of the optical frequency. For this reason, the dispersion must be controlled by averaging after detecting the frequency of the beat signal 10-100 times, thus prolonging the processing time.
Furthermore, since the frame of the reference pulsed light is long, a considerable period of time elapses from the time at which the frequency of the beat signal is detected until the time at which control is fed back to the local light source, thus degrading the short-term stability.